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INVETOR LELAND e. THOMPSON L. E. THOMPSON AUTOMATIC FREQUENCY CONTROLFiled May 22, 1946 Feb. 22, 1949.

ATTORNFV flex Klystron ID.

Patented Feb. 22, 1949 s PATENT lOFFICE AUTOMATICfrmicun:Nor CONTROLLeland E.` Thompson, Merchantville, NMI., assgnor to Radio Corporationof America, a corporation of Delaware .f Application May 22, 194s,serilNo. 671,454

` 9 ciaims. (01.250-36) This invention relates tota method of landapparatus for automatically controlling thel frequency of oscillatorsvgeneratingvery high frequency oscillations, of theorder of 500megacyclesand higher;

A detailed description of theinvention follows `in conjunction with adrawing `vvherein Fig. 1

illustrates one embodimentof the automatic frequency controlr system ofthe invention, and Fig. 2 illustrates a modification ofrafportion ofFig. 1 Which can be substitutedfor that portion to the left of verticaldash line X--Y.

Referring to the drawing there-is shown afreex Klystron oscillator IWhose frequency is to bestabilized, a reference tuned circuit I'22.of'highQ (low loss) `Whose frequency is to be compared` with the frequencyof the oscillations produced by the oscillator I0, and a frequencycorrecting circuit comprising a crystal rectifier I4, a resistor II`across which the voltages rectified by the crystal I4 appear, .anaudioamplii-ler f I8, 20, a balancedphase detector systemincluding apair of rectifiers 22 anda pairof equal resistors 24, and a directcurrent amplier 26.

.. The reiiex Klystron oscillator I'llwhose frequency is to becontrolled is illustrative of. Aany suitable oscillator generating veryhigh frequency oscillations. By Way-of example, this oscillator maygenerate oscillations of the order of I 4000 megacycles, althoughthisfrequency is given `may be employed either in a transmitter or a.receiver. v

The referencetuned circuit i2 maybe any suitable cavity resonator butYis here shoWnlas a concentric linetuned circuit having an "outerconductor O and an adjustable inner. conductor I. The inner Vand outerconductors areconnected together by a metallic end plate Thev lengthofthe linner conductor controls the frequency of the tuned circuit I2and is adjustable by means of a tuning knob N. It is preferred that theconductors of thistuned circuit :be made Aof Invar,

which is alow temperature coeflicient .materiaL andA coated with highlyconducting electrical material such as silver, gold or copper'. ...This

"tuned :circuit is .preferably sealed against humidity so that kitsresonant frequency is very f: constant and its stability comparable tothatof a quartz crystal. .The Q of the tuned circuit I2 is made to bevery high, on the orderl of 2000 to 4000, by Wayof example.

'A ."Acrystal rectifier. I4 is Vcoupled very loosely merely. by Way ofillustration. Oscillatorl is shown as comprising an evacuated envelopehaving therein a cathode K for producing a beam of electrons, a toroidalcavity resonator M of suitable dimensions and having apertures in bothsides to permit the beam of electrons from the vcathode K to passtherethrough, and a reflector or repeller electrode R. The voltage onthe repeller electrode is of negative polarity relative to the cathodeand repels the electrons passing through-the toroidal resonator M. Theelectrons in this Klystron oscillator move to and fro through the cavityresonator M until they are nally collected. The dimensions of the cavityresonator M and the voltage on the reflector electrode R determine to alarge extent the frequency of the oscillations generated by the re- Suchan oscillator is Well known in the art and no claim is made to it perse.

Output from the Klystronoscillator I0 is taken froma loop D located inthe interior of the cavity resonator M. This loop is coupled to acoaxial transmission line TLV which extends to an antenna (not shown).The system as illustrated to the reference resonator I2 by means of con-.centric .line TL' and loop B. Loop B enters the f .interior of theresonator I2 and is coupled to the electromagnetic eld Within thisconcentricrline tuned circuit.

The frequency of the low' loss resonator I2 is modulated or changed upand down at an audio frequency rate by means ofla polarized vibratingplunger or Vibrator circuit 8. This Vibrator comprisesa permanent magnet1 located-Within a coil 9 which is` coupled to a source'of 60 cyclealternating current. This 60' cycle alternating currentmay, by Way ofillustration, have a voltage of 6 volts. A suitable Vibrator springr 5locatediin the interior of the concentric line res- *onator I2y is fixedat oneend to a spacer y3 and Ahas Aits other end secured to an ironbutton I. This iron button vibrates with the spring 5 and moves in andout of an aperture in the outervconductor Il. Between the solenoid 9 andthe permarient magnet "I there is provided a brass tube I I one end ofwhich is sealed ofi' by a suitable cap I3 and the other end of WhichissealedA off` by a "suitable non-magnetic material I5 such aspolystyrene, in order to prevent moisture or' air from coaxial resonatorI2, together With the oscillator entering the interior of the outerconductor 0 through the aperture. It is preferred that the I0 andthecircuit in which the crystal rectifier -Il! is located, be located in atemperature Ycontrol boxq not shown.

The lcrystal rectifier I4 is located in a suitable `metallic' stub Sland is preferably tuned. yOne theresonant frequency of this resonator.

terminal of the crystal rectifier is connected to the inner conductor ofline TL. The line TL is coupled at one end by way of loop B to theresonator I2 and at the other end coupled by way of loop C to the lineTL extending to the antenna. The other terminal of the crystal rectierI4 is connected to the by-pass condenser I"I at the closed end'of thestub. 'I'his'by'i'a'ss condenser comprises a pair of metallic plates onopposite sides of and suitably spaced from the end of stufb S. This lastterminal of thecrystal rectiiieris connected via lead I9 to one end of ahresistor I6 whose other end is connected to ground through l a meter M.Lead I9 is also connected to 91160 cycle or 120 cycle audio amplifiercomprising a pair of vacuum tube triode stages, I8 `and 2 0 ari rangedin cascade.

The output of the 60 cycle audio amplifirls,

20 is coupled to a balanced phase detector circuit comprisingra'pair ofrectiers `22 connected'to opposite ends of the secondary winding of anIaudio transformer f2I. Itfshould be understood, however, thattherectiers 22 may be replaced by diodes, vifit is so desired. 'A pairoffequal value resistors 24 are connected in series across the 'outputof the balanced phase detector. AAlternating current in the 'form of 60cycle energyv is supplied Vto Ithe balancedphase detector throughan'audiotransformer 23 and a variable phase'adjuster 25. Output from thebalanced phase detector is taken by way of leads 21 and fed Vto a directcurrent amplifier 2E. This directeurrent amplier kfurnishes negative'voltage to the lr'eiiector electrode R of the Klystron oscillator 'I0of a'magnitude and sense necessary to correct the frequency of theKlyst'ron oscillator in order to cause'thisifrequency to conform withthe frequency of the reference tuned circuitIZ.

'If the system isused as a'transmitter, modulation potentials maybeisupplied to the oscillator 'I0 by way ofaf'transformer 29. Thismodulation may Ybein the form 4of an audio or aradio -frequencysub-carrier. AIf-audio modulation .is femployed, the ftransformer'29should be an iron 'core transformer whereasif -a Y radio frequencysub-carrier frequency is employed the transformv 'er` 29 should be anair core transformer. The (radio frequency sub-carrier modulation may bejamplitude or frequency 1 modulation.

ever typeof modulation 'is employed, whether :audio or radio frequencysub-carrier, the vresult l 'will be a frequency modulation of theoscillations -produced by the reflex Klystron I0.

In the operation ofthe system, the permanent magnet lI of thevibrator l8normally exerts force to pull the; lower end of the vibrator springtoward it.` The application'ofthe 60 cycle alternating current ,to thecoil 9 causes the flux produced in the vibrator duringone-halffcycle tooppose thel pulling force of the permanent'magnet andhence permit thelspring-5 to move back toward the inner conductor I of the resonator I2.During the other half cycle, the flux produced by the coil 9 aids 'thatproduced by the magnet and pulls the .vibrator spring towards thepermanent magnet.V The movement ofthe spring toward and `away from theyinner conductor changes the capacity ofthe resonator I2y and hencechanges This oQQurs at a rate of 60 `cycles per second. The alternating.current output,v from the crystal rectifier Il is either near vzeroJorhas a frequency;of 12,0 cycles when the oscillator I0 is 1 ",fintunewithhthe: coaxial line resonator I2, `and this output increases or'the"60 cycle component hold the frequency of the oscillations producedby Vth'e 'bscillatbr I 0 to thefequency 'of' the reference resonator I2.Very loose coupling is employed between the reference resonator I2 andthe oscillator I Il, and 4also between the reference resonator I2 andthecrystal rectifier I4.

`Wh' en the oscillator III is exactly in tune with th'rereferenceresoriator I2, a direct current volt- "ag'e will appear acrossthe crystal rectifier load resistor; I ,In this condition, there will beeither no alternating voltage across resistor I6 or there 'wiil existacross this resistor the second harmonic of the 60 c yclel modulatingsupply.y 'Ihe vibrator' 'causes' the resonantfreque'ncyof the resonatorI2 to changefrom'one" side-'tothe otherside ofthe 1 resonant frequencyofthe resonator I21at a' period of'60 cycles'persecond, and thisproducesan alternating current variation of theY crystal current 'ata period Vof120c`ycle'sfper second. This 120 cycle current'is'amplied by the' audioamplifier I8, 20 andapplied to the balanced phase detector but this120`cycle current' does not produceany direct current outputpotentialfrom the balanced Whichphase'detector. If'thelfrequency of theoscillator IIJ drifts in a dre'ctinso' as to be higher than the desiredfrequency' ofthe oscillator I0, then a cycle voltagelcompone'nt willappear across resistor I6. This 60 cycle voltage or 'component isdeveloped in the' crystal rectifier I4 and is'amplified by a two=stageaudio amplifier I8, '20and rfed tothe'bal'anc'ed detectors22. Thepotential pro- "duced by the` balanced phase detector in this case willvbe a Vdirect currento'utput which may be in a 'negative direction.It"'should vbe 4'noted' that the detectors'or copperoxidelrectiers'22are excited in parallel by'theisamerGOcycle alternating cur- .resistors'24 are l'qual and opposite in direction, so

thatthes'uin 'is zero. However, 'when''cycle negative.

The outputfromthefbalanc'ed phase detector I'iacitor 3l filter'outthecycle component'and "applythe resultantdirect current voltage to thedirect current amplifier-'26. A cnangein anode Yvoltage thenA occurs inthe "direct current' amplier-26 andthis change isfapplied to" therepeller electrode R'of th'efoscillatr` I'Ilin order to correct :thefrequency of"the;oscillations'produced bythe reex Klystronr oscillator.

It should be-understood that if the'frequency ofV oscillations from' theoscillator ldrifts down in frequency, then v'thephase of the 60cycle'voltageappearingyacross'resistorI6 is opposite or reversed 'from.the phase ofthe 60 cyclevoltage f' appearing across thisrsameresistorlwhen the oscillator frequency drifts upward. The `polarity ofthe direct current voltage inthe output of the balanced phase detectoris reversed or opposite' in .the case of the oscillator frequency I 0drifting the power fed to the antenna, and this small percentage kofradio frequency energy is fed to the crystal rectifier I4. The resonatorI2 variably absorbs energy from the loop B, depending upon -the resonantfrequency of the concentric line resonator. When the frequency of theconcentric line resonator I2 is the same as the transmitter frequency,then the resonator I2 absorbs a maximum and the radio frequency currentrectified by the crystal I4 is a minimum. When the resonator I2 absorbsenergy, it does this at the expense of the energy being rectified by thecrystal rectifier I4. When the resonant frequency of the resonator I2changes to either side of the frequency of oscillations produced by thetransmitter or oscillator I0, then less energy is absorbed by theresonator I2 and hence more current is rectified by the crystalrectifier I4.

An important feature of the invention is the location of the crystalrectifier which is between the reference resonator I2 and the highfrequency oscillator I0. An advantage of this location lies in the factthat the portion of the line TL' between the crystal rectifier I4 andthe loop C can be made to be any length, for example ve feet, more orless, This is because the distance between the upper terminal of thecrystal rectifier I4 and the closed end of the stub S is made to be axed dimension, in order to provide an impedance match to the line TL'over a wide frequency range. By way of illustration, in one embodimentof the invention actually tried out in practice, there was employed afrequency range of 3900 megacycles to 4400 megacycles produced by theoscillator I0, and the foregoing impedance match was a good one overthis entire range of frequencies.

Another important feature of the invention lies in the use of thevariable phase adjustment 25 for adjusting the phase of the 60 cyclealternating current voltage fed to the balanced detector. Thisadjustment, which is in the form of a tap on a resistance, permitsadjustment of the phase of the 60 cycle alternating current voltage withrespect to the alternating current voltage output from the crystalrectifier. This assures maximum control of the transmitter and preventsundesired frequency changes of the transmitter oscillator I0 which mightotherwise occur in the event the phase of the two alternating currentvoltages to be compared are different from the necessary value.

It should be noted that the cathode of the direct current amplifier 2Bis connected to a potentiometer 33. This potentiometer 33 is connectedin series to a fixed high resistance 34. The value of resistance 34 ischosen so that the bleeder current through 33 and 34 is several timesthe anode current of 26, so that objectionable degeneration does notoccur in the direct current amplifier 26.

The potentiometer control 33 is used to initially adjust the Klystronrepeller voltage to the correct value, preferably before the vibrator inresonator I2 is started.

If desired, the crystal I4 may be coupled directly to the cavityresonator circuit I2, rather than be positioned in the location shown inthe drawing, in which case it is not necessary totune the crystalcircuit and the transmission line .between the cavity resonator andtheoscillation generator tube can be very short.A Such an arrangementisshown in Fig. 2 which is a fragmentary view showing only those parts ,of`the system necessary` for an understanding of the modification andwhich can be substituted for that portion of Fig. 1 to the left ofvertical dash line X-Y. The same elements in both Figs. 1 and 2 areidentified by the same reference characters, and the connections I9 andTL ofFig. 2 extend to the same circuits as the same numbered leads inFig. 1. The means for utilizingthevariable voltage produced by thebalanced detecting system for controlling the frequency of operation ofthe high frequency oscillator is not shown Vin Fig. 2 in the interest ofsimplifying the drawing but is identical with that shown in Fig. 1.

What is claimed is: c

1. In combination, a generator of high .frequency Waves, a stable lowloss tuned resonator circuit tuned to a desired frequency of operation,a vibrator circuit coupled to said resonator circuit for changing thefrequency of operation of said resonator circuit, a source of low audiofrequency current coupled to saidvibrator circuit and energizing thesame to operate at the frequency of said source, a circuit couplingsaidgenerator to said' resonator circuit for excitingy said resonatorcircuit with waves derived from said generator, a rectier within ametallic shell coupled to said last coupling circuit and located betweensaid resonator circuit and said generator, a balanced phase detectingsystem coupled to said rectifier and to said source of low audiofrequency current, said balanced detecting system producing a variablevoltage as the frequency of said generator varies with respect to themean frequency of operation of said variably tuned resonator circuit,and means utilizing the variable voltage so produced for controlling thefrequency of operation of said generator. l

2. In combination, a generator of Very high frequency waves, a stablelow loss tuned resonator circuit tuned to a desired frequency ofoperation, a vibrator circuit coupled to said resonator-circuit forchanging the frequency of operation of said resonator circuit, a sourceof low audio frequency current coupled to said vibrator circuit andenergizing the same to operate at the frequency of said source, acircuit coupling said generator to said resonator circuit for excitingsaid resonator circuit with waves derived from said generator, arectifier coupled to said last coupling circuit and located between saidresonator circuit and said generator, a balanced phase detecting systemcoupled to said rectifier and to said source of low audio frequencycurrent, a variable phase adjusting circuit located between said phasedetecting system and said source of low audio frequency current, saidbalanced detecting system producing a variable voltage as the frequencyof said generator varies with respect to the mean frequency of operationof said variably tuned resonator circuit, and means utilizing thevariable voltage so produced for controlling the frequency of operationof said generator.

3. In combination, a generator of very high frequency waves, a stablelow loss tuned resonator circuit tuned to a desired frequency ofoperation, a vibrator circuit coupled to said resonator circuit forchanging the frequency of operation of said resonator circuit, saidvibrator circuit including a permanent magnet. a coil surround- 7 ingsaid magnet, and a vibratorfspringfatone end ofsaid magnet and vlocatedwitlilnsaid resonator, asour'ce of low audio frequency current coupledto the coil. of said vibrator circuit and energizing the sameto'oper'ate at the frequency of said source, a circuit coupling saidgenerator to said resonator circuit for exciting said resonator circuitWith waves derived from said generator, a rectifier Within a metallicshell coupled to said last icoupling circuit and located between said"resonator circuit and said generator, a balanced V'phase detectingsystem coupled to said rectifier and to said source of low audiofrequency current, said balanced detecting system producing a variablevoltage as the frequency of said generator varies with vrespect t thevm'eanfrequency of operation of said variably tuned resonator circuit,and means utilizing the variable voltage so produced for controlling thefrequency of operation of said generator.

4. In combination, a generator 'of very high frequency Waves, a stablelow loss tuned resonator circuit tuned to a desired frequency ofoperation, a vibrator circuit coupled to said resonator circuit forchanging the frequency of operation of said resonator circuit, a sourceof low audio frequency current coupled to said vibrator circuit andenergizing the same to operate at the frequency of said source, acircuit coupling said generator to said resonator circuit for excitingsaid resonator circuit with waves derived from said generator, arectifier within a metallic shell coupled to said last coupling circuitand located between said resonator circuit and said generator, abalanced phase detecting system coupled to said rectifier and to saidsource of low audio frequency current, said balanced phase detectingsystem including a pair of copper oxide rectiers, a variable phaseadjuster located between said rectiers and said source of low audiofrequency current, said balanced detecting system producing a variablevoltage as the frequencyl of said generator varies with respect to` themean frequency of operation of said variably tuned resonator circuit,and means utilizing the'variable voltage s`o produced for controllingth'efrequency of operation of said generator. l

5. In combination, a generator of oscillations, a stable circuit tuned-to a desired frequency of oscillations generated by saidgeneraton'vsaid stable circuit being coupled to said generator so as tobe excited by oscillations-generated'by Ysaid generator, a source ofrelatively low audio frequency waves, means responsive tofwaves fromsaid low audio frequency vsource for cyclically varying the tuning ofsaid stablefcircuitabout a desired frequency of operation, ay rectifierconnected to a point intermediate the ends of an energy transfer circuitextending' between said stable circuit and said generator forrectifyingy waves derived from said generator, said rectifier `beingpositioned within a metal-lic 'stub 'and 7'caquency of operation, andinstrumentalities resp'onsive to the voltage produced-by 'said balanceddetectingsystem to reduce the departure in frequency of operation ofsaid generator away from the desired frequency of operation.

16. In combination, a generator of very high 'frequency Waves, a stablelow loss tuned resonator circuit tuned to a desired frequency ofoperation, a vibrator circuit coupled to said resonator circuit forchanging the frequency vof 'operation of said resonator circuit, asource of low audio frequency current coupled to said vibrator circuitand energizing the same to operate at the yfrequency of said source, acircuit coupling said generator to said resonator circuit for excitingsaid resonator circuit with waves derived from said generator, arectifier coupled to said reso- 'nator circuit, a balanced phasedetecting system coupled to said rectifier and to said source of lowaudio frequency current, a variable phase adjusting circuit locatedbetween said phase detecting system and said source of low audiofrequency current, said balanced detecting system producing a variablevoltage as the frequency of said generator varies with respect to themean frequency of operation of said variably tuned resonator circuit,and means utilizing the variable voltage so produced for controlling thefrequency of operation of said generator.

7. In combination, a generator of very high frequency waves, a stablelow loss tuned resonator circuit tuned to a desired frequency ofoperation, a vibrator circuit coupled to said resonator circuit forchanging the frequency of operation of said resonator circuit, saidvibrator circuit including a permanent magnet, a coil surrounding saidmagnet, and a vibrator spring at one end of said magnet and locatedwithin said resonator, a source of low audio frequency current coupledto the coil of said vibrator circuit and energizing the same to operateat the frequency of said source, a circuit coupling said generator tosaid resonator circuit for exciting said resonator circuit with Wavesderived from said generator, a rectifier coupled to said resonatorcircuit, a balanced phase detecting system ycoupled to said rectifierand to said source of low audio frequency current, said balanceddetecting system vproducing a Variable voltage as the frequency of saidgenerator varies with respect to the mean frequency of operation of saidvariably tuned resonator circuit, and means utilizing the variablevoltage so produced for controlling the frequency of operation of saidgenerator.

8. In combination, a generator of high frequency Waves, a `stable lowloss tuned resonator circuit tuned to'a desired frequency of operation,

l a Vibrator circuit coupled to said resonator circuit 'for changing thefrequency of operation of said resonator circuit, a source of low audiofrequency current coupled to said vibrator circuit and energizing thesame to operate at the frequency oi said source, a circuit coupling'saidgenerator to said resonator circuit for exciting Vsaid resonator circuitwith waves derived from said generator, 'a crystal rectifier Within ametallic shell directly coupled t-o said resonator circuit, a balancedphase detecting system coupled to said rectifier and to said source oflow audio frequency current, said balanced detecting system producing avariable voltage as the frequency of said generator varies with respectto the mean frequency 'of operation of said variably tuned resonatorcircuit, and means utilizing the variable voltage so produced vforcontrolling the frequency of operation of said generator.

9. In combination, a generator of very high frequency waves, a stablelow loss tuned resonator circuit tuned to a desired frequency ofoperation, a vibrator circuit coupled to said resonator circuit forchanging the frequency of operation of said resonator circuit, a sourceof low audio frequency current coupled to said vibrator circuit andenergizing the same to operate at the frequency of said source, acircuit'coupling said generator to said resonator circuit for excitingsaid resonator circuit with waves derived from said generator, a crystalrectifier directly coupled to said resonator circuit, a balanced phasedetecting system coupled to said rectifier and to said source of lowaudio frequency current, said balanced phase detecting system includinga pair of copper oxide rectiers, a variable phase adjuster locatedbetween said rectiers and said source of low audio frequency current,said bal- REFERENCES vCITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Number Name Date 2,312,919 Litton Mar.A 2, 19432,356,414 Linder Aug. 22, 1944 2,393,284 Brown Jan. 22, 1946 2,404,568Dow July 23, 1946 2,408,425 Jenks Oct. 1, 1946

